Sacrificial zinc anodes



March 6, 1962 J. R. M EWAN 3,024,183

SACRIFICIAL ZINC ANODES Filed Dec. 14, 1959 FIG. I

Inventor JOHN R. Mac EWAN by: M

United States Patent Ofilice 3,024,183 Patented Mar. 6, 1962 3,024,183SACRIFICIAL ZHJC ANODES John R. MacEwan, Deep River, Ontario, Canada,assignor to The Consolidated Mining and smelting Company of CanadaLimited, Montreal, Quebec, Qanada, a company of Canada Filed Dec. 14,1959, Ser. No. 859,378 7 Claims. (Cl. 204--197) This invention relatesto a method of producing sacrificial or consumable zinc anodes for usein inhibiting corrosion by galvanic action, and sacrificial zinc anodesproduced thereby.

The use of sacrificial zinc anodes is well known for the protection ofmetal structures and equipment such as heat exchangers, pipe lines, shiphulls, storage tanks and the like. In some instances, anodes are made inthe form of cast blocks, or slabs, with steel inserts to provideelectrical contact between the anode and the metal structure to beprotected, and also to provide means for fastening the anode to themetal structure to be protected.

In other instances, long, narrow anodes are required or preferredbecause of their particular characteristics such as high current outputper unit of volume, ease of installation and more streamlined design.Such long, narrow anodes are also made by casting methods in whichmolten zinc is poured into moulds around a metal core, or insert. Thecore extends the length of the anode and provides the necessaryelectrical contact and also the means for attaching the anode to thestructure.

Casting methods are not completely satisfactory for the production oflong, narrow zinc anodes. The cast anodes are not flexible and thereforethey are not easily adapted to types of service in which curved orcoiled anodes are desired. Anodes are required to be made in a number ofdifferent sizes which means that the manufacturer must maintain a largesupply of suitable moulds. Furthermore, casting in moulds is notpractical for lengths greater than about 5 or 6 feet.

It is known that zinc can be formed in long, narrow shapes by extrusion,but a major difiiculty arises in the formation by extrusion of zincanodes with the insert or core required for electrical contact andsupport. This core must be effectively bonded to the zinc along thelength of the anode to ensure that a positive and permanent electricalcontact between the zinc and the core material is maintained throughoutthe life of the anode. If the bond is imperfect, corrosive solution, forexample, sea water, can penetrate along the junction of the core and thezinc metal when the anode is in service and the resultant corrosionproducts will insulate the core from the zinc and thus render the anodeinelfective for its in tended purpose.

The core member must extend the length of the anode as otherwise theanode would break as it became corroded in service, thereby severing theelectrical connection between anode and protected structure, and theanode would cease to function.

I have found that the difiiculties encountered in the manufacture,installation and use of sacrificial zinc anodes of elongated shape canbe overcome by coextruding zinc and aluminum. According to my invention,I form zinc anodes by extruding a composite billet comprised of acylindrical block of zinc with an aluminum core.

I have also found that the required positive and permanent bond betweenzinc anode metal and core material can be obtained by applying a thinprime coating of molten zinc to an aluminum core, and thereafter,casting molten zinc around the zinc-coated aluminum core to form thecomposite zinc-aluminum billet. This composite billet, which is in theform of a thick zinc sheath around an aluminum core, can then beextruded to form elongated stock of desired cross-sectional shape foruse, when cut into suitable lengths, as sacrificial or consumable zincanodes with aluminum cores.

The aluminum core is treated to ensure proper bonding between zinc andaluminum before the zinc sheath is cast around it. This treatmentinvolves the steps of fluxing and dipping the core. In the fluxing step,a clean, degreased aluminum core is coated with a suitable flux foraluminum surfaces and is then heated to melt the flux to form acontinuous film of flux on the surface of the core. Excess flux can beremoved by wiping. Suitable fluxes for aluminum surfaces include thosebased on zinc chloride and containing fluorides, such as those used innormal aluminum fluxing operations. The flux should not be allowed toabsorb water. Melting of the flux can be eflected by heating theflux-coated core in a gas flame or in an oven at a temperature betweenabout 1000 F. and 1050 F.

In the dipping step, the hot, fluxed core is dipped in a bath of moltenzinc maintained at a temperature above the melting point of zinc,preferably between 900 F. and 950 F. This dipping step ensures theformation of a continuous, uniform coating of zinc on the aluminum corewith a zinc-aluminum alloy at the interface between the aluminum and thezinc.

In the casting step, the freshly dipped, hot core is placed in a billetmould and molten zinc is poured around it to form a composite billet.The zinc is cast into the mould at a relatively high temperature,preferably between 1000 F. and 1050 F. Preferably the aluminum coreshould not be allowed to cool between the clipping and casting steps,and it should not be exposed to the air for more than a few minutes.

A suitable billet mould is illustrated in the accompanying drawing inwhich:

FIGURE 1 is an elevation in section; and

FIGURE 2 is a top plan View.

Referring to the drawing, the vertical cylindrical mould 10 is formed ofconventional material such as cast iron or graphite. A recess 11 isformed in the base of the mould. This recess is preferablyconcentrically positioned, and is adapted to receive the lower end ofthe rod-shaped aluminum core 12. A supporting frame 13 extends from thetop of the mould and is adapted to engage the upper part of the aluminumcore and thus hold the core firmly in a vertical position while themolten zinc is poured into the mould through access ports 14.

The composite billet is withdrawn from the mould after the zinc hascooled and solidified, and can be stored, if desired, before subsequenttreatment.

Sacrificial zinc anodes in accordance with my invention are producedfrom such billets by extrusion. The extrusion step is effected withconventional extruding appanatus which involves a chamber adapted toreceive the billet, a piston or ram adapted toapply pressure against thebase of the billet and a die of dmired shape at the forward end of thebillet through which the metal is extruded. The extruded metal is cutinto desired lengths for use as anodes. Usually the first few and thelast few inches of the composite extrusion are cut off and discarded asthe core diameter may not be uniform at the extreme ends.

I have found, however, that the core diameter is consistently uniformthroughout the remainder of the stock, and that the reduction indiameter of the aluminum core in the composite billet to form thealuminum core in the extruded anode stock is substantially the same inproportion as the reduction in diameter of the billet to form the anodestock.

The invention is illustrated by the following example: A cylindricalaluminum core 14 inches long and 1 /2 inches in diameter was cleaned,degreased with trichlorethylene, and coated with a flux paste made up of70% of a zinc chloride soldering flux and 30% normal propyl alcohol. Inthe present example, the flux was a commercial product sold as AlcoaSoldering Flux No. 66. The core was dried to remove the alcohol andheated in a gas flame to melt the flux and form a continuous film offlux on the surface of the core. The hot, flux-coated .core was thendipped in a bath of molten zinc maintained at a temperature of about 930F. When the core was immersed in the molten zinc, fuming and bubblingoccurred. The reaction subsided in about 30 seconds and the core wasthen withdrawn. This dipping step provided a continuous adherent film orcoating of zinc on the aluminum core.

Immediately after the dipping step, the hot, zinc-coated core was placedin a billet mould of the type illustrated in the accompanying drawing,and molten zinc at a temperature of about 1025" F. was poured into themould. After the billet had cooled and solidified, it was removed fromthe mould and trimmed to remove protruding ends of the aluminum core.The resulting composite billet was 12 inches long and 4 inches indiameter.

The billet was extruded through a conventional cylindrical die at 500 F.and 370 to 420 tons pressure to produce cylindrical anode stock 0.84inch in diameter with an aluminum core 0.3 inch in diameter. Theextrusion was about 20 feet long and the core diameter was consistentlyuniform throughout the length of the extrusion, except for about 12inches at each end. The first and last 12 inches of the extrusion werecut off and discarded. The remaining 18 feet of anode stock was cut intolengths of 6 feet for use as sacrificial or consumable zinc anodes.

Prior to use, parts of the aluminum core are exposed for the purpose ofmaking an electrical connection with the metal structure the anode isdesigned to protect.

In the anodes made according to the foregoing example, the aluminum usedwas the commercial grade of about 99.9% purity and the zinc used wasSpecial High Grade (99.99%) alloyed with about 0.4% aluminum. However,other suitable compositions can also be used. For example, high purityzinc which contains less than 0,0015% iron is generally satisfactory andother zinc alloys are also known in the art. Similarly, other grades ofaluminum can also be used.

The bond between the zinc and the core in the anodes of the presentinvention has been tested physically and examined under a microscope.The microscopic examination revealed that the zinc and aluminum werealloyed at the junction or interface of the core and anode metal, thealloy blending into the Zinc on the outer side and into the aluminum onthe inner side of the interface. The physical tests showed that the bondbetween the core and anode metal was strong and adherent, and wasimpervious when subjected to dye penetrant tests, such as the Zyglotest.

The bond between the aluminum core and the zinc is essential forsatisfactory operation of the anode, and it is also essential forsatisfactory extrusion of the composite billet. Unless a bond isobtained over the whole interface between the aluminum rod and zincmetal sheath of the composite billet, the extrusion step will notproduce a continuous core of uniform diameter. For example, a compositebillet was prepared by drilling a hole in a solid zinc cylinder andcasting aluminum into the resulting cavity. When this billet wasextruded, it was found that the aluminum core was discontinuous, andthat extrusion lubricant had been forced between the aluminum and zinc,thus preventing metal to metal contact. Also, unsatisfactory resultswere obtained with composite billets formed by casting molten zincaround dipped aluminum rods which had not been fluxed or around fiuxedrods which had not been dipped.

The sacrificial or consumable zinc anode produced by the presentinvention possesses several important advantages. It can be producedrelatively inexpensively from readily available materials. It can beused with advantage to protect steel structures such as ships hulls,storage tanks, hot water tanks, underground installations, and the like,and also for protecting aluminum structures. An aluminum core hasimportant advantages over a steel core when zinc anodes are used for theprotection of aluminum from corrosion as steel would be cathodic toaluminum and it would be essential to remove steel cores before all thezinc was consumed. Otherwise, the aluminum structure would corrode whenthe zinc had been consumed as the aluminum would act as a sacrificialanode for the steel core. When using zinc anodes with aluminum cores,this problem with aluminum structures does not arise.

Also, from the point of view of fabrication, an aluminum core ispreferable to a steel core as a composite billet of zinc and aluminumcan be easily extruded. Furthermore, extruded zinc anodes are ductile,and can be bent or coiled, if desired, for ease of installation or tomeet particular service requirements.

The method is particularly advantageous for making long, narrow anodes,but can be used also with any extrudable cross section.

The anodes and the cores, or inserts, can be made any desired shape byselection of suitable extrusion dies, and by the use of suitably shapedaluminum cores when preparing the composite billet. The anode stock canbe cut into any desired length, usually from a few inches to 12 feet ormore, depending on the required service.-

It will be understood, of course, that modifications can be made in thepreferred embodiment of this invention described hereinabove withoutdeparting from the scope of the invention as defined by the appendedclaims.

What I claim as new and desire to protect by Letters Patent of theUnited States is:

1. The improved method of producing sacrificial zinc anodes whichcomprises the steps of coating an aluminum core with a film of Zinc toform a zinc-aluminum alloy at the interface, casting molten zinc aroundthe zinccoated aluminum core to form a composite zincaluminum billet,cooling said billet below the melting temperature of zinc, andthereafter extruding said billet to form sacrificial zinc anode stockwith an aluminum core.

2. The improved method of producing sacrificial zinc anodes whichcomprises the steps of coating an aluminum core with a flux for aluminumsurfaces, coating said fluxcoated core with an adherent film of zinc toform a zincaluminum alloy at the interface thereof, casting molten zincaround the zinc-coated aluminum core to form a composite zinc-aluminumbillet, cooling said billet below the melting temperature of zinc, andthereafter extruding said billet to form sacrificial zinc anode stockwith an aluminum core.

3. The method according to claim 2 in which the flux is a zinc chloridepaste.

4. The improved method of producing sacrifical zinc anodes whichcomprises the steps of coating an aluminum core with a flux for aluminumsurfaces, dipping said fluxcoated core in a bath of molten zinc andwithdrawing said core therefrom whereby said core is coated with anadherent film of zinc to form a zinc-aluminum alloy at the interfacethereof, casting molten zinc around the zinccoated core while the coreis still hot from the dipping step to form a composite zinc-aluminumbillet with a zinc-aluminum alloy at the interface, and thereafterextruding said billet to form sacrificial zinc anode stock with analuminum core.

5. The method according to claim 4 in which the flux is a zinc chloridepaste.

6. A sacrificial zinc anode which comprises a zinc extrusion having analuminum core extending throughout the length thereof, said extrusionhaving a zinc-aluminum alloy at the interface of the aluminum and thezinc.

7. A sacrificial zinc anode which comprises a zinc extrusion bonded toan aluminum core, the bond at the interface of the zinc and the aluminumbeing in the form of a zinc-aluminum alloy.

References Cited in the file of this patent UNITED STATES PATENTS GrebeAug. 9, 1949 Grebe et a1 Aug. 9, 1949 Brooks et al Feb. 21, 1956Robinson July 1, 1958

1. THE IMPROVED METHOD OF PRODUCING SACRIFICAL ZINC ANODES WHICHCOMPRISES THE STEPS OF COATING AN ALUMINUM CORE WITH A FLIM OF ZINC TOFORM A ZINC-ALUMINUM ALLOY AT THE INTERFACE, CASTING MOLTEN ZINC AROUNDTHE ZINCCOATED ALUMINUM CORE TO FORM A COMPOSITE ZINCALUMINUM BILLET,COOLING SAID BILLET BELOW THE MELTING TEMPERATURE OF ZINC, ANDTHEREAFTER EXTURDING SAID BILLET TO FORM SACRIFICIAL ZINC ANODE STOCKWITH AN ALUMINUM CORE.